1. Field of the Invention
The present invention generally relates to jet engines and other applications including combustion in an environment of high speed gas flow and, more particularly, to so-called scramjets, ramjets and jet engine afterburners.
2. Description of the Prior Art
Efficient and reliable operation of supersonic combustion ramjet (scramjet) engines and ramjet engines (which operate similarly but with sub-sonic flow regimes) and other applications needing combustion in high speed flow regimes (such as jet engine afterburners) relies on rapid and predictable mixing of fuel with ambient air, reliable ignition and flameholding and enhancement of the combustion reaction. Ignition and flameholding is particularly difficult at flow velocity regimes approaching and above the speed of sound. Depending on the fuel choice and flight speed, different ignition arrangements including cavities or ramps (which are well-known geometries for protection for an ignition source) and pyrophoric compounds have been used for ignition and flameholding. Similarly, shock waves and other flow anomalies within an engine at such flow velocity regimes have held the degree of mixing of fuel and air (and/or other combustion gases whether ambient or applied to the engine in the manner of a fuel) hereinafter referred to simply as fuel mixing to levels presumably much less than optimum. Upstream fuel injection is generally considered to be required in order to provide time for atomization and mixing of the fuel in a high speed gas flow prior to ignition but which is very limited due to restrictions on size of an engine usable in a practical aircraft. That is, at supersonic flow rates, fuel injected upstream from the igniting arrangement will reach and pass the igniting arrangement in a very short time well before mixing and atomization (which increases surface and reaction area of fuel droplets) has taken place to an optimum degree since the fuel injection and ignition locations must necessarily be less than the engine length which, in turn, must necessarily be no greater than the aircraft length. Additionally, there appears to be a trade-off with the distance of the fuel injection point in advance of the igniter which affects mixing and atomization and the likelihood of the fuel reliably reaching the igniter. Therefore optimization of the combustion reaction, especially for wide range operation, is difficult.
Moreover, all three of these requirements are currently addressed by separate mechanisms or arrangements, each of which has tended to be bulky and of significant weight, contrary to what is desirable in an aircraft engine. Thus, attempts to improve the efficiency of any of them in regard to their function generally tends to increase bulk, weight or both while yielding only marginal improvements in engine performance.
Use of an electric plasma torch for ignition and flameholding is known. However, simply using a plasma torch as an ignition source does not mitigate the problems of obtaining good fuel mixing or combustion enhancement. Further, in such a system, as with ramps, cavities and pyrophoric compounds, use of known plasma torches is not significantly controllable to regulate the performance of the combustion system. Further, difficulties and delays in achieving ignition due to insufficient atomization and mixing presents a substantial safety hazard. Additionally, approaches to improvement of ignition, atomization, mixing and combustion enhancement known in the art have required structures which intrude into the high speed air flow; causing further disturbances and shock waves that may disrupt, for example, uniformity of fuel distribution (which often tends to follow the wavefronts of shock waves).
While these and other problem are particularly significant in scramjet engines, they also appear to greater or lesser degrees in other applications for developing high temperature and stable ignition in a high velocity flow regime such as jet afterburners or ramjets (where the flow is subsonic) and treatment or refinement of bulk materials as is industrial processes depending on a flow of heated air).